1. Field of the Invention
The present invention relates to a light amplifier, particularly to an optical fiber amplifier of controllable gain doped with a rare earth element such as erbium . The erbium-doped fiber will be hereinafter referred to as EDF.
2. Description of the related art
A wide variety of devices has been proposed in the field of EDF light amplifiers. For example, Japanese Patent Laid-open No. 22687/95 (hereinafter, referred to as reference 1) describes an EDF light amplifier capable of performing a constant-gain control even when an input of a light signal is absent.
In reference 1, a light signal and a reference signal that has a wavelength slightly different from that of the light signal are entered into the EDF which has been placed in a state of population inversion by a pumping light supplied from a laser diode. Stimulated emission is caused in the EDF by both the light signal and the reference signal. A part of the output from the EDF is optically filtered to extract the reference signal component, which is in turn converted to an electric signal. A laser driving circuit controls the output of the laser diode in response to the electric signal. Controlling the output of the laser diode so that the ratio of the filtered light signal to the reference signal is constant yields a constant-gain control. In this way, a constant-gain control become feasible even when an input of a light signal is absent.
Japanese Patent Laid-open No. 97941/97 describes an EDF light amplifier in which a light signal component of a desired wavelength included in a wavelength-multiplex light signal is controlled to have a constant output level. Hereinafter, this light amplifier is referred to as reference 2.
In the light amplifier of reference 2, the light signal component of the desired wavelength is modulated at a prescribed frequency beforehand. The wavelength-multiplex light signal including the light signal of concern is entered into the EDF that has been placed in a state of population inversion, thereby causing a stimulated emission of radiation.
A part of the output of the EDF is converted to an electric signal, from which an electric signal of the prescribed frequency is filtered. The detected level of the filtered electric signal is supplied to a control circuit, which controls the intensity of the pumping light corresponding to the light signal of the prescribed frequency . In this way, the light signal of a prescribed wavelength is controlled so as to have a desired signal level.
Japanese Patent Laid-open No. 327104/93 (hereinafter, referred to as Reference 3) describes a light amplifier of a low noise figure.
In this light amplifier, a pumping light creates population inversion in the EDF; a light signal fed to the EDF gives rise to stimulated emission of the EDF, causing amplification of the light signal; the amplified light signal is attenuated by a high-loss optical fiber; and a part of the output of the high-loss optical fiber is fed back to control the intensity of the pumping light so as to adjust the output of the light amplifier. Due to the highloss optical fiber, the level of the pumping light becomes high compared to a light amplifier of the same output level having no high-loss optical fiber. The high intensity of the pumping light allows a low noise figure.
A light amplifier has been known from Japanese Patent Laid-open No. 364790/92 (hereinafter, referred to as Reference 4), in which the output of the light amplifier is adjusted to be constant by a plurality of pumping light sources.
In this amplifier, an input light signal is amplified by an EDF that is activated (placed in a state of a population inversion) by a pumping light fed from the plurality of pumping light sources. A part of the output light signal is branched to be converted to a first electric signal. The first electric signal is compared with a reference signal to generate an error signal. The error signal is branched into the plurality of error signal components corresponding to the number of the pumping light sources. Each error signal component is supplied to each of output control circuits. Each output control circuit controls the corresponding pumping light source. Each of output control circuits receives, as a feed-back signal, a second electric signal that is produced by photoelectric conversion of a branched part of the pumping light.
Each output control circuit controls the pumping light in two modes, a short-time response mode and a longtime response mode. In the long-time response mode, the output control circuit controls the pumping light source so as to minimize the error signal component. In the shorttime response mode, the output control circuit controls the pumping light source so as to minimize the deviation of the second electric signal from the error signal component. In this way, constant-output control is attained.
Another light amplifier has been proposed in which a depolarized light source is used to check polarization dependence of the gain associated with the pumping light.(Japanese Patent Laid-open No. 308547/94: This reference is referred to as Reference 5). In one specified embodiment, a passive polarization scrambler is used to depolarize the pumping light. The scrambler is connected with an output of the single-frequency laser that generates the pumping light.
The above-described references are mainly directed to the methods of performing a constant-output control, in which the intensity of the pumping light is controlled by an output of the fiber light amplifier.
However, there has been developed a fiber light amplifier based on other concept. In this fiber light amplifier, spontaneous emission (hereinafter, referred to as SE) emitted from an optical fiber doped with rare earth element, such as erbium, is detected when the optical fiber is in the state of population inversion, and the pumping light source is controlled by the detected value of the SE.
As is known well, the probability per photon that an input light signal induces stimulated emission, i.e., macroscopically, the ratio of the intensity of the output light signal with respect to the intensity of the input light signal, or the gain of the fiber light amplifier, is proportional to the number of the rare earth atoms in a state of population inversion.
Since the intensity of the SE as well is proportional to the number of the rare earth atoms in a state of population inversion, the intensity of the SE is proportional to the gain of the fiber light amplifier.
For this reason, control of the SE by controlling the pumping light implies a control of the gain by controlling the pumping light.
The fiber light amplifier based on the SE concept above is shown in FIG. 1.
Referring now to FIG. 1, a light signal applied to input terminal A is transmitted to EDF 5 through optical branching circuit 6 and isolator 7. Monitor (photo-detector) 8 monitors the part of the light signal that has branched at optical branching circuit 6.
A pumping light emitted by pumping light source 2 is entered into EDF 5 through optical multiplexer 9. The population inversion produced in EDF 5 by the pumping light causes to amplify the input light signal. The amplified light signal is taken out from output terminal B through optical multiplexer 9 and optical isolator 10.
EDF 5 is wound in a shape of a coil and is fixed. Photo-detector 4 for detecting the SE emitted from EDF 5 is arranged radially above the side surface of the EDF coil, as is shown in FIG. 2.
Photo-detector 4 detects the SE emitted from EDF 5. The output of photo-detector 4 is supplied to output-control section 3, which in turn controls the bias current of pumping light source (laser diode) 2 in response to the intensity of the SE, thereby adjusting the gain of the light amplifier to be constant.
A problem in a light amplifier of the earlier development described above has been in the structure of the fiber amplifier that the EDF is wound to shape a coil, above the side surface of which photo-detector 4 is settled, as is depicted in FIG. 2.
In this structure, photo-detector 4 detects an SE emitted only from the outermost wound EDF. Since the intensity of the SE emitted from the EDF depends on the longitudinal position of the fiber, monitoring of the SE emitted from only a certain special position does not provide information about the SE sufficient to carry out a precise constant-gain control. For this reason, it is necessary to conduct monitoring of the SE over the entire length of the EDF.